1. Field of the Invention
The present invention relates generally plastic or polymer-based mirrors, and specifically to a lightweight and durable synthetic resin mirror resistant to warpage and a method for the manufacture thereof.
2. Description of Related Art
Mirrors typically have a multilaminate configuration. In particular, mirrors are typically formed by selectively depositing a series of compounds on a glass substrate material. These layers generally include a reflective layer and a protective back-coat layer covering the reflective layer. The reflective layer is commonly formed from a thin film of aluminum, chromium, rhodium, or silver. In industrial applications, aluminum is typically used in place of silver due to its high reflectivity and low cost. The protective back-coat layer serves a multiplicity of functions, such as protecting the reflective layer from humidity. This function is required as the reflective layer, especially if it is formed from aluminum, is easily corroded by moisture. Since the substrate material upon which the reflective layer is deposited is often permeable to moisture, it is important that the protective back-coat layer be substantially impermeable to moisture in order to provide an effective encasement for the reflective layer. The back-coat layer also serves as a mechanical barrier to, for example, impact damage from airborne particulate matter. A properly configured and applied back-coat layer thus assists to provide a durable mirror.
Due to the high production costs related to glass mirrors, significant research has been undertaken to develop a durable, low-cost plastic mirror employing a synthetic resin substrate material. Furthermore, due to their shatter-proof nature of synthetic resin mirrors, their use is preferred in automobiles over conventional glass mirrors in order to improve the safety of the automobile. The primary focus of this research has been in connection with dynamically stable and substantially optically clear thermoplastic or thermoset resins, such as polymethyl methacrylate (PMMA). As a result of these efforts, a method for sequentially depositing an aluminum reflective material and an impermeable back-coat layer on a resin substrate material has been developed.
The main problem associated with synthetic resin mirrors is their significantly limited operational service life resulting from warpage or distortion of the mirrors due to the hygroscopic properties of thermoplastics or thermoset resins. Unlike their glass counterparts, mirrors formed with a thermoplastic or a thermoset resin as their substrate material gradually absorb moisture from the surrounding atmosphere. Over time, the moisture so absorbed corrodes the reflective layer. Further, the absorption of moisture, coupled with variations in other climatic conditions, causes the thermoplastic or thermoset resin to expand and contract. Compounding these problems is the fact that the back-coat layer is, typically, not affected by humidity or other climatic conditions. The back-coat layer thus acts to prevent the smooth linear expansion and contraction of the thermoplastic or thermoset resin substrate. Furthermore, the moisture permeability of the various coatings applied to both sides of the synthetic resin substrate often lead to different amounts of moisture being absorbed by the opposing surfaces of the synthetic resin substrate, thus resulting in uneven expansion and contraction on both sides of the substrate. These conditions all interact to produce distortion to the image produced by the reflective layer of the plastic mirror and a related loss of optical clarity. As the mirror ages, this degradation only becomes more acute.
In order to reduce the susceptibility of synthetic resins to hygroscopic effects, it has been proposed that a hardening material be applied to the thermoplastic or thermoset resin substrate before deposition of the reflective layer. Currently organosilicon polymers are the preferred hardening material. These polymers are preferred due to their ability to provide protection against impact damage and their high optical clarity when fully cured. Although organosilicon polymers are the best available material for this purpose, these polymers are not totally impermeable to water. Thus, although partially effective, these polymers do not provide a complete remedy to all of those issues related to the use of a thermoplastic or thermoset resin substrate material in connection with a mirror apparatus.
A need exists for a mirror apparatus that does not suffer from the foregoing disadvantages and limitations. In particular, a need exists for a mirror apparatus formed using a thermoplastic or thermoset resin substrate that will remain substantially unaffected by ambient environmental conditions.
The foregoing shortcomings and disadvantages of the prior art are alleviated by the present invention that provides a polymer-based mirror that is resistant to mechanical distortion resulting from climatic and hydrodynamic conditions. The polymer-based mirror includes a substrate or transparent element formed from a synthetic thermoplastic or thermoset resin, such as polymethyl methacrylate or the like. The resin substrate has an anterior surface and a posterior surface. A tie-bond layer is typically applied to all of the exposed surfaces of the resin substrate.
Following deposition of the tie-bond layer, a surface-hardening layer is coated on at least the anterior surface of the resin substrate. This layer may consist of one or more layers of various materials which form a surface-hardening layer substantially impermeable to water. A surface-hardening layer formed of the following layers has been found to provide a desired level of moisture permeability for the anterior surface of the synthetic resin substrate: 500 to 1200 angstroms of SiO, preferably 750 angstrom; 300 to 1200 angstroms of SiO2, preferably 550 angstrom; and, 600 to 1400 angstroms of Zv(iPv)2, preferably 725 angstrom. A surface-hardening layer may also be applied to the posterior surface of the synthetic resin substrate, where the posterior surface-hardening layer preferably comprises 300 to 1200 angstroms of SiO2, preferably 550 angstrom; and, 600 to 1400 angstroms of Zv(iPv)2, preferably 725 angstrom.
A reflective layer of a composition substantially resistant to moisture is deposited on the posterior side of the resin element. The reflective layer comprises a series of materials sequentially deposited onto the posterior surface of the treated resin substrate. A reflective layer formed from the following layers exhibits the desired reflectance, moisture permeability, and durability for the polymer-based mirror of the present invention: 500 to 1200 angstroms of SiO, preferably 750 angstroms; 700 to 1500 angstroms of aluminum, preferably 1200 angstroms; 500 to 1200 angstroms of SiO, preferably 750 angstroms; 600 to 1400 angstroms of Zv(iPv)2, preferably 725 angstroms; and 300 to 1200 angstroms of Sio2, preferably 550 angstroms. The reflective layer of the invention is preferably formed on the synthetic resin substrate via a vacuum deposition technique. A protective back-coat layer is then deposited over the reflective layer to encase the outer surface of the reflective film layer. When the surface-hardening layer is also applied to the posterior surface of the resin substrate, the back-coat layer can also encase the surface-hardening layer as well as the reflective layer. A multi-layer weather-resistant coating may optionally be applied to the anterior surface of the polymer-based mirror in order to increase the weatherability and durability of the mirror.
Overall, the polymer-based mirror of the present invention has a multilaminate configuration including sequentially deposited layers of organic and inorganic materials. The polymer-based mirror of the present invention exhibits superior moisture resistance as compared to conventional aluminum, chromium, and rhodium coated mirrors. The present invention further provides a mirror that is easily and economically produced.